As is well known to those of ordinary skill in the art, annealing is a heat treatment process wherein a material is heated to a suitable temperature for a period of time, and then cooled. In the semiconductor industry, wafer materials, such as silicon, are annealed, so that dopant atoms, such as boron, phosphorus or arsenic, etc., can diffuse into substitutional positions in the crystal lattice, resulting in changes in the electrical properties of the semiconducting material.
The introduction of dopants in a semiconductor is commonly achieved by ion implantation. Dopant ions such as boron, phosphorus or arsenic, etc, are generally created from a gas source. When implanted in a semiconductor, each dopant atom can create a charge carrier in the semiconductor (hole or electron, depending on if it is a p-type or n-type dopant), thus modifying the conductivity of the semiconductor in its vicinity.
Semiconductor devices made from silicon or other semiconductor materials are routinely made with ion implantation because they can be made with planar technology. Ohmic contacts with a lower contact resistance can be fabricated, and the doping concentration and profiles can be more closely controlled. The dopants can be electrically activated routinely by heating the silicon (or other semiconductor material) in a non-oxidizing atmosphere to a temperature of roughly 1000° C. Being able to ion implant dopants has many advantages such as enabling more planar technology and lowering the contact resistance. Implantation activation in the III-V semiconductors such as gallium arsenide (GaAs) or gallium nitride (GaN) is more difficult than with elemental semiconductor materials because the group V element; e.g., arsenic or nitrogen, has a relatively large vapor pressure, and it will evaporate preferentially at the temperatures required to activate the implants. If left unprotected during the activation anneal, the device structure will be destroyed or significantly degraded. This problem has been solved for GaAs by depositing a silicon nitride (Si3N4) layer that acts as an annealing cap for the annealing temperatures of 800-900° C. and prevents the arsenic (As) from escaping. The deposited layer can then be preferentially etched off of the GaAs with hydrofluoric acid without attacking the GaAs. This problem has also been solved for SiC using a graphite or AlN/BN annealing cap to prevent the preferential evaporation of silicon.
Si3N4 and SiO2 annealing caps for GaN have had only mixed success because they do not adhere well and can be punctured or blown off by the large nitrogen partial pressure that exists at the temperature required for activation. GaN cannot be routinely implanted because the nitrogen evaporates preferentially at the temperatures required to activate the implanted dopants. Si3N4, cannot be used as an annealing cap for GaN because (a) higher activation temperatures are required (>1200° C.), (b) it does not adhere well to GaN, and (c) it cannot withstand the higher N pressures, as blow holes can form in Si3N4 annealing cap.
Likewise, sputtered or pulsed laser deposition (PLD) deposited ALN films have had mixed success because they do not adhere to the GaN surface making it possible for the nitrogen to escape at the elevated temperatures. Researchers have also attempted to rapid thermal anneal (RTA) their samples in the hope that the kinetics for the evaporation of N would be too slow for it to respond, but considerable topological damage is created. Annealing under an N2 pressure that exceeds the partial pressure of N2 in equilibrium with GaN is also extremely challenging given that the partial pressure is approximately 100 MPa at 1500 degrees K.
The single sputtered AlN film disclosed in the publication by J. C. Zolper, et al., J. Electron. Mater. 27, 179 (1998) does not adhere well, and it contains pores through which the N from the GaN can escape. As used herein, sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic ions. As used herein, sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by sputtering, i.e. ejecting, material from a “target,” i.e., source, which then deposits onto a substrate or layer.
The PLD deposited AlN film disclosed in K. A. Jones, et al., J. Mater. Sci. and Eng. B61-62, 281 (1999) (hereby incorporated by reference) did not adhere sufficiently as well, as is shown in FIG. 1A. Epitaxial AlN films grown by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) adhere better, but they crack due to the lattice mismatch between the GaN and AlN as shown in FIG. 1B. The single epitaxial AlN film grown at the normal growth temperature for AlN films (approximately 1100° C.) as disclosed in the publication by C. J. Eiting, et al, Appl. Phys. Lett. 73, 3875 (1998), did not provide good coverage because it cracked since it was not sufficiently pliable and because it does not have the strain relief mechanisms that a low temperature AlN film provides. The single AlN film deposited at low temperature by molecular beam epitaxy (MBE) disclosed by J. A. Fellows, et al., Appl. Phys. Lett. 80, 1930 (2002) is not strong enough to withstand the relatively large N pressures from the GaN.
Accordingly, there exists a need for an annealing cap that has better adherence, that is mechanically stronger and still capable of completely covering the sample, that is stable at temperatures in excess of 1200° C., and that is capable of being selectively etched off at the end of the anneal.